A torsional damper for absorbing torsional vibration resulting from torque fluctuation is arranged in an automobile at a location, for example, where an output shaft of an internal combustion engine is coupled to an input shaft of a transmission. As such a torsional damper, a known torsional damper includes a friction-type torque limiter, which slips when an excessive torque is input to prevent excessive load from acting on each part of a mechanism coupled to the torsional damper.
A dry torque limiter is often used as such a friction-type torque limiter arranged in a torsional damper. The dry torque limiter holds a dry friction plate with a predetermined pressure, and transmits torque through the frictional force generated by the friction plate.
In such dry torque limiter, however, rusting of the friction plate may increase the tolerable transmission torque, which is the maximum torque that can be transmitted without causing slippage. When the tolerable transmission torque increases, slippage does not occur when the transmission of torque should be stopped. As a result, the internal combustion engine and the transmission may not be protected from excessive load.
Japanese Laid-Open Patent Publication No. 2008-274969 discloses a torsional damper that prevents such changes in the tolerable transmission torque. This torsional damper is entirely accommodated in a case, and lubricating oil is filled in the case. This immerses a spring of the damper and a friction plate of a torque limiter in the lubricating oil.
In detail, as shown in FIG. 7, the torsional damper described in Japanese Laid-Open Patent Publication No. 2008-274969 includes a case 3 having a rear cover 1 and a front cover 2. A torque limiter 5, which includes a friction plate 4, and a damper 7, which includes a spring 6, are arranged in the case 3. The case 3 is filled with lubricating oil to lubricate the torque limiter 5 and the damper 7.
When employing such a structure, the lubricating oil functions to prevent the friction plate 4 from rusting. This prevents changes in the tolerable transmission torque that would be caused when the friction plate 4 rusts.
In the torsional damper described in Japanese Laid-Open Patent Publication No. 2008-274969, however, the spring 6 of the damper 7 is immersed in the lubricating oil. Thus, when the temperature of the lubricating oil is low, the viscosity of the lubricating oil inhibits deformation of the spring 6. As a result, the effect of the damper 7 for absorbing torsional vibration may not be fully exhibited.